MOSFETs with channels on nothing and methods for forming the same

ABSTRACT

A method includes performing an epitaxy to grow a semiconductor layer, which includes a top portion over a semiconductor region. The semiconductor region is between two insulation regions that are in a substrate. The method further includes recessing the insulation regions to expose portions of sidewalls of the semiconductor region, and etching a portion of the semiconductor region, wherein the etched portion of the semiconductor region is under and contacting a bottom surface of the semiconductor layer, wherein the semiconductor layer is spaced apart from an underlying region by an air gap. A gate dielectric and a gate electrode are formed over the semiconductor layer.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.13/436,322, entitled “MOSFETs with Channels on Nothing and Methods forForming the Same,” filed on Mar. 30, 2012, which application isincorporated herein by reference.

BACKGROUND

With the increasing down-scaling of integrated circuits,Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) becameincreasingly smaller, with increasingly shorter gates. This requires thejunctions, particularly the junctions between lightly doped source/drainregions and pocket regions, to be shallower. However, due to thediffusion of the implanted impurities, it is very difficult to reducethe depth of the junction to about 50 Å for Short Channel Effect (SCE)control.

What makes the reduction of the junction depths more difficult is thatfor smaller MOSFETs, the pocket regions need to have higher impurityconcentrations. However, with heavier pocket implantations, severaladverse effects may be resulted. For example, although the SCE controland the Drain-Induced Barrier Lowering (DIBL) may be better, the carriermobility of the carriers in the channel region is degraded, resulting insmaller device drive currents. To solve these problems,Silicon-On-Nothing (SON) MOSFETs have been proposed. SON MOSFETs haveair-gaps under channel regions. With the air-gaps, the SCE of the SONMOSFETs is improved, and leakage currents can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 8B are cross-sectional views, top views, and perspectiveviews of intermediate stages in the manufacturing of aMetal-Oxide-Semiconductor Field Effect Transistor (MOSFET) in accordancewith some exemplary embodiments, wherein the MOSFET has a channel onnothing; and

FIGS. 9 through 13 illustrate cross-sectional views, top views, andperspective views of intermediate stages in the manufacturing of aMOSFET in accordance with some alternative exemplary embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare illustrative, and do not limit the scope of the disclosure.

A Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) with achannel on nothing and the methods of forming the same are provided inaccordance with various exemplary embodiments. The intermediate stagesof forming the MOSFET are illustrated. The variations of the embodimentsare discussed. Throughout the various views and illustrativeembodiments, like reference numbers are used to designate like elements.

Referring to FIG. 1, substrate 20, which may be a portion of asemiconductor wafer, is provided. Substrate 20 may be a semiconductorsubstrate. Substrate 20 may be a bulk substrate, or may have asilicon-on-insulator structure. In some embodiments, substrate 20 is asilicon substrate, although it may also comprise silicon germanium(SiGe), silicon carbon (SiC), or the like. Insulation regions such asShallow Trench Isolation (STI) regions 22 are formed in a top portion ofsubstrate 20. STI regions 22 may be formed by recessing semiconductorsubstrate 20 to form openings, and then filling the openings withdielectric materials. STI regions 22 may include two neighboring regionshaving their sidewalls 22A facing each other, with a portion ofsubstrate 20 between, and adjoining, the two neighboring STI regions 22.

Referring to FIG. 2, the portion of substrate 20 that is between twoneighboring STI regions 22 is removed, forming opening 24. In someembodiments, the bottom of opening 24 is level with the bottoms of STIregions 22. In alternative embodiments, the bottom of opening 24 may belower than or higher than the bottoms of STI regions 22.

Referring again to FIG. 3A, epitaxy region 26 is formed in opening 24,for example, using Selective Epitaxial Growth (SEG). Epitaxy region 26may be formed of a semiconductor material, which may comprise a group IVmaterial such as Si, Ge, C, or combinations thereof. In some exemplaryembodiments, epitaxy region 26 is formed of substantially puregermanium, for example, with a germanium percentage greater than about95 percent. In alternative embodiments, epitaxy region 26 comprisesSiGe, which may be expressed as Si_(1-z)Ge_(z), wherein z is the atomicpercentage of germanium in the silicon germanium, and z may be greaterthan 0, and may be equal to or less than 1. When z is equal to about 1,SiGe layer 26 is formed of substantially pure germanium. In someexemplary embodiments, z is between about 0.1 and about 1. In yet otherembodiments, epitaxy region 26 comprises a III-V compound semiconductormaterial comprising, but is not limited to, GaAs, InP, GaN, InGaAs,InAlAs, GaSb, AlSb, AlAs, AlP, GaP, combinations thereof, andmulti-layers thereof. Epitaxy region 26 may also be formed of a II-VIcompound material. The top surface of epitaxy region 26 may be levelwith, higher than, or lower than, the top surface of STI regions 22. Insome embodiments, epitaxy region 26 is grown to a level higher than thetop surfaces of STI regions 22. A planarization such as a ChemicalMechanical Polish (CMP) is performed to level the top surface of epitaxyregion 26 with the top surfaces of STI regions 22.

FIG. 3B illustrates a perspective view of the structure shown in FIG.3A, wherein the cross-sectional view in FIG. 3A is obtained from theplane crossing line 3A-3A in FIG. 3B. In some embodiments, epitaxyregion 26 has a dog-bone shape including narrow portion 26A, and wideportions 26B connected to the opposite ends of narrow portion 26A. WidthW1 of narrow portion 26A may be between about 5 nm and about 20 nm.Width W2 of wide portions 26B may be greater than about 20 nm, orgreater than about 40 nm. One of ordinary skill in the art will realize,however, that the dimensions recited throughout the description aremerely examples, and may be changed to different values. Although wideportions 26B are illustrated as having a rectangular shape, they mayalso have other shapes. For example, wide portions 26B may havetrapezoid shapes, with the narrow sides of the trapezoids connected tonarrow portion 26A.

Referring to FIG. 4, STI regions 22 are recessed. The recessing depth D1may be greater than about 5 nm, for example. Accordingly, a portion ofepitaxy region 26 protrudes above the top surfaces of the adjacent STIregions 22. Next, as shown in FIG. 5, epitaxy semiconductor layer 28 isgrown on the top surface and sidewalls of epitaxy region 26. Epitaxysemiconductor layer 28 may be a substantially conformal layer, and hencethe sidewall portions and the top portion have substantially the samedepth. The epitaxy is selective, so that epitaxy semiconductor layer 28is not grown on STI regions 22. Thickness T1 of epitaxy semiconductorlayer 28 may be smaller than about 40 nm, and may be between about 5 nmand about 40 nm, for example. Epitaxy semiconductor layer 28 includesnarrow portion 28A formed on the top surface and sidewalls of narrowportion 26A of epitaxy region 26. Furthermore, epitaxy semiconductorlayer 28 includes wide portions 28B (refer to FIG. 7B) formed on the topsurfaces and sidewalls of wide portions 26B of epitaxy region 26.

In the embodiments illustrated in FIGS. 2 through 5, epitaxy region 26is formed to replace the portion of substrate 20, which portion isbetween opposite sidewalls of STI regions 22. In alternativeembodiments, the steps shown in FIGS. 2 through 3 are skipped. Instead,the STI recessing step is performed on the structure shown in FIG. 1,and the resulting structure is similar to what is shown in FIG. 5,except that the illustrated epitaxy region 26 is not epitaxially grown,and instead, is a portion of the original substrate 20, which may be asilicon substrate, for example.

FIG. 6 illustrates the further recessing of STI regions 22, whereinrecessing depth D2 may be between about 5 nm and about 40 nm. Portionsof the sidewalls of epitaxy region 26 are thus exposed.

Next, as shown in FIG. 7A, an etching is performed to etch the exposedportion of epitaxy region 26. The etchant is selected to attack epitaxyregion 26, while the etching to epitaxy semiconductor layer 28 and STIregions 22 is minimal, if any. Accordingly, the materials of epitaxyregion 26 and epitaxy semiconductor layer 28 are also selected to have ahigh etching selectivity, and an appropriate etchant may be selectedcorrespondingly. In some exemplary embodiments, epitaxy semiconductorlayer 28 is formed of silicon, and epitaxy region 26 is a formed ofgermanium. The corresponding etchant may be hydrogen fluoride (HCl). Inalternative exemplary embodiments, epitaxy semiconductor layer 28 isformed of Si_(1-y)Ge_(y), and epitaxy region 26 is formed ofSi_(1-x)Ge_(x), with x>y. The corresponding etchant may also be HCl. Inyet alternative exemplary embodiments, epitaxy semiconductor layer 28comprises InGaAs, and epitaxy region 26 comprises InP. The correspondingetchant may be HCl. In yet alternative exemplary embodiments, epitaxysemiconductor layer 28 comprises InP, and epitaxy region 26 comprisesInGaAs. The corresponding etchant may be a combination of H₃PO₄ andH₂O₂. In yet alternative exemplary embodiments, epitaxy semiconductorlayer 28 comprises GaSb, and epitaxy region 26 comprises InAs. Thecorresponding etchant may be a combination of citric acid (C₆H₈O₇) andH₂O₂. In yet alternative exemplary embodiments, epitaxy semiconductorlayer 28 comprises InAs, and epitaxy region 26 comprises GaSb. Thecorresponding etchant may be NH₄OH.

As shown in FIGS. 6 and 7A, after the etching, the narrow portion 26Ahas at least its top portion removed, and hence narrow portion 28A ofepitaxy semiconductor layer 28 is separated from the underlying epitaxyregion 26 (or substrate 20 if epitaxy region 26 is fully removed) by airgap 30. It is observed that air gap 30 includes a portion higher thanthe top surfaces of STI regions 22, and a portion between STI regions22.

FIG. 7B illustrates a perspective view of the structure shown in FIG.7A, wherein the cross-sectional view in FIG. 7A is obtained from theplane crossing line 7A-7A in FIG. 7B. It is observed that wide portions26B of epitaxy region 26 are recessed from the sidewalls. However,center portion 26C of wide portions 26B remain not etched. FIG. 7Cillustrates a cross-sectional view of one of the center portions 26C,wherein the cross-sectional view is obtained from the plane crossingline 7C-7C in FIG. 7B. Accordingly, epitaxy semiconductor layer 28 issupported by the center portion 26C of epitaxy region 26, althoughnarrow portion 28A of epitaxy semiconductor layer 28 is fully suspendedabove air gap 30.

Next, as shown in FIGS. 8A and 8B, MOSFET 100 is formed, with narrowportion 28A of epitaxy semiconductor layer 28 forming the channel regionof MOSFET 100. FIGS. 8A and 8B illustrate a cross-sectional view and atop view, respectively, with the cross-sectional view in FIG. 8Aobtained from the plane crossing line 8A-8A in FIG. 8B. As shown inFIGS. 8A and 8B, MOSFET 100 includes gate dielectric 32, gate electrode34, and gate spacers 36 on the sidewalls of gate electrode 34. Referringto FIG. 8B, gate dielectric 32 and gate electrode 34 may overlap narrowportions 28A, and may no overlap wide portions 28B (FIG. 8B), of epitaxysemiconductor layer 28. Referring to FIG. 8A, gate dielectric 32 isformed on the sidewalls and the top surface of channel region 28A. Thematerial of gate dielectric 32 may include silicon oxide, siliconnitride, or high-k dielectric materials such as Hf-containingdielectrics or other kinds of high-k materials. Gate electrode 34 may beformed of polysilicon, metals, metal silicides, and/or the like.

Next, as also shown FIG. 8B, source and drain regions 38 are formed,wherein the formation process may include, for example, an implantationstep, which is performed after the formation of gate spacers 36. In someembodiments, an additional epitaxy layer, which is also illustratedusing reference numeral 38, may be grown on wide portions 28B of epitaxysemiconductor layer 28. The additional epitaxy layer also forms parts ofthe source and drain regions 38.

Referring to FIG. 8A again, narrow portion 28A of epitaxy semiconductorlayer 28 forms the channel region of MOSFET 100. Since channel region28A includes a top portion 28A1, and two edge portions 28A2 below andconnected to the opposite sides of top portion 28A1, the respectiveMOSFET 100 is a Fin Field-Effect Transistor (FinFET).

FIGS. 9 through 13 illustrate cross-sectional views of intermediatestages in the formation of a MOSFET in accordance with alternativeembodiments. Unless specified otherwise, the materials and the formationmethods of the components in these embodiments are essentially the sameas the like components, which are denoted by like reference numerals inthe embodiments shown in FIGS. 1 through 8B. The details of the likecomponents shown in FIGS. 9 through 13 may thus be found in thediscussion of the embodiments shown in FIGS. 1 through 8B. The processis briefly discussed herein.

The initial steps of these embodiments are essentially the same as shownin FIG. 1. Next, referring to FIG. 9, epitaxy region 26 is formed. Thetop-view shape of epitaxy region 26 may be found in FIG. 11B. The topsurface of epitaxy region 26 is level to the top surface of the STIregions 22, which are on the opposite sides of epitaxy region 26. Theportion of substrate 20 between the opposite STI regions 22 is recessedslightly to form recess 40. Next, as shown in FIG. 10, epitaxysemiconductor layer 28 is formed in the recess through epitaxy.Thickness T2 of epitaxy semiconductor layer 28 may be between about 5 nmand about 40 nm, for example.

In alternative embodiments, the formation of epitaxy region 26 isskipped. Instead, a top portion of semiconductor substrate 20 betweenopposite sidewalls of STI regions 22 (please refer to FIG. 1) isrecessed slightly to form recess 40 as in FIG. 9, and epitaxysemiconductor layer 28 is grown in recess 40 and from the exposedportion of substrate 20. The resulting structure is essentially the sameas the structure shown in FIG. 10, except that epitaxy region 26 isinstead a portion of original substrate 20.

Next, as shown in FIGS. 11A and 11B, which are a cross-sectional viewand a perspective view, respectively, STI regions 22 are recessed, sothat epitaxy semiconductor layer 28 is higher than the resulting STIregions 22. The portions of the sidewalls of epitaxy region 26 are alsoexposed. As shown in FIG. 11B, epitaxy region 26 includes narrow portion26A, and wide portions 26B connected to opposite ends of narrow portion26A. Epitaxy semiconductor layer 28 may include narrow portion 28A, andwide portions 28B connected to the opposite ends of narrow portion 28A.Width W2 is greater than width W1.

Next, as shown in FIGS. 12A and 12B, an etching is performed to removethe top portions of narrow portion 26A of epitaxy region 26.Accordingly, air gap 30 is formed under the narrow portion 28A ofepitaxy semiconductor layer 28. As shown in FIG. 12B, wide portions 26Bare recessed from their sidewalls, and center portions 26C of wideportions 26B are not etched. Accordingly, epitaxy semiconductor layer 28is supported by center portions 26C, and the narrow portion 28A issuspended over air gap 30. The etchant for performing the etching isrelated to the materials of epitaxy semiconductor layer 28 and epitaxyregion 26. Alternatively, if epitaxy semiconductor layer 28 is formed onsubstrate 20, with no epitaxy region 26 formed, the etchant is selectedto attach epitaxy semiconductor layer 28 without attacking substrate 20.The exemplary materials and the respective etchants are discussed in theembodiments shown in FIGS. 1 through 8B.

FIG. 13 illustrates the formation of MOSFET 100, which includes gatedielectric 32 and gate electrode 34. A top view of the MOSFET 100 inFIG. 13 is essentially the same as shown in FIG. 8B. Similarly, sourceand drain regions 38 and gate spacers 36 are also formed. As shown inFIG. 13, in these embodiments, channel region 28A is planar, and hencethe respective MOSFET 100 is a planar MOSFET.

It is observed that in the MOSFET 100 as shown in FIGS. 8A, 8B, and 13,channel region 28A have bottom surfaces higher than the top surface ofthe adjacent STI regions 22. Furthermore, the top portions of air gaps30 are higher than the top surfaces of STI regions 22. As a result, therespective gate electrodes 34 may have portions lower than the topsurfaces of air gaps 30. In FIG. 8A, air gap 30 includes a portionextending between the opposite edge-portions 28A2, and edges 30A of airgap 30 are substantially aligned to the respective edges 22A of STIregions 22. Furthermore, in FIGS. 8A and 13, edges 30B of air gap 30 areover and aligned to STI regions 22, and have a greater space from eachother than edges 30A. Air gap 30 insulates channel region 28A fromsubstrate 20, and hence the leakage current of MOSFET 100 is reduced,and the short channel effect is improved. In the resulting MOSFET 100,air gap 30 may be filled with air, or may be vacuumed.

In accordance with embodiments, a device includes a semiconductorsubstrate, and a channel region of a transistor over the semiconductorsubstrate. The channel region includes a semiconductor material. An airgap is disposed under and aligned to the channel region, with a bottomsurface of the channel region exposed to the air gap. Insulation regionsare disposed on opposite sides of the air gap, wherein a bottom surfaceof the channel region is higher than top surfaces of the insulationregions. A gate dielectric of the transistor is disposed on a topsurface and sidewalls of the channel region. A gate electrode of thetransistor is over the gate dielectric.

In accordance with other embodiments, a device includes a semiconductorsubstrate, and insulation regions at a top portion of semiconductorsubstrate. The insulation regions have two sidewalls facing each other,wherein the opposite sidewalls are spaced apart from each other by aspace. A channel region formed of a semiconductor material is disposedover the semiconductor substrate and aligned to the space. An air gap isdisposed underlying and aligned to the channel region, wherein the airgap includes edges substantially aligned to respective ones of the twosidewalls of the insulation regions. A gate dielectric is on a topsurface and sidewalls of the channel region. A gate electrode is overthe gate dielectric. The channel region, the gate dielectric, and thegate electrode form parts of a transistor.

In accordance with yet other embodiments, a method includes performingan epitaxy to grow a semiconductor layer. The semiconductor layer has atop portion over a semiconductor region. The semiconductor region isbetween two insulation regions that are in a substrate. The insulationregions are recessed to expose portions of sidewalls of thesemiconductor region. A portion of the semiconductor region is etched,wherein the etched portion of the semiconductor region is under andcontacting a bottom surface of the semiconductor layer. Thesemiconductor layer is substantially not etched, and the semiconductorlayer is spaced apart from an underlying region by an air gap. Themethod further includes forming a gate dielectric over the semiconductorlayer, and forming a gate electrode over the gate dielectric.

In accordance with some embodiments of the present disclosure, a methodincludes performing an epitaxy to grow a semiconductor layer, whichincludes a top portion over a semiconductor region. The semiconductorregion is between two insulation regions that are in a substrate. Themethod further includes recessing the insulation regions to exposeportions of sidewalls of the semiconductor region, and etching a portionof the semiconductor region, wherein the etched portion of thesemiconductor region is under and contacting a bottom surface of thesemiconductor layer, wherein the semiconductor layer is spaced apartfrom an underlying region by an air gap. A gate dielectric and a gateelectrode are formed over the semiconductor layer.

In accordance with alternative embodiments of the present disclosure, amethod includes performing an epitaxy to grow an epitaxy semiconductorlayer over a semiconductor region, wherein the semiconductor regionincludes a middle portion and end portions on opposite sides of themiddle portion. The semiconductor region is etched to remove the middleportion of the semiconductor region to form a recess, wherein a centerpart of each of the end portions of the semiconductor region remainsafter the etching. A gate dielectric and a gate electrode are formedover the semiconductor layer.

In accordance with alternative embodiments of the present disclosure, amethod includes performing a first recess to recess STI regions to havea first top surface, wherein a top portion of a semiconductor region inthe STI regions is over the first top surface. An epitaxy semiconductorlayer is formed on a top surface and sidewalls of the top portion of thesemiconductor region. A second recess is performed to recess the STIregions to have a second top surface, wherein a bottom end of theepitaxy semiconductor layer is over, and is spaced apart from, thesecond top surface, with an intermediate portion of the semiconductorregion exposed. An intermediate portion of the semiconductor region isetched to form an air gap, wherein the air gap is between the epitaxysemiconductor layer and a remaining bottom portion of the semiconductorregion. The method further includes forming a gate dielectric over amiddle portion of the epitaxy semiconductor layer, with the gatedielectric overlapping the air gap, and forming a gate electrode overthe gate dielectric.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: performing an epitaxy togrow a semiconductor layer, wherein the semiconductor layer comprises atop portion over a semiconductor region, and wherein the semiconductorregion is between two insulation regions that are in a substrate;recessing the insulation regions to expose portions of sidewalls of thesemiconductor region; etching a portion of the semiconductor region,wherein an etched portion of the semiconductor region is under andcontacting a bottom surface of the semiconductor layer, wherein thesemiconductor layer is spaced apart from an underlying region by an airgap; forming a gate dielectric over the semiconductor layer; and forminga gate electrode over the gate dielectric.
 2. The method of claim 1,wherein the method further comprises: before performing the epitaxy,performing an additional recessing to recess the insulation regions,wherein after the additional recessing, a top portion of thesemiconductor region protrudes above top surfaces of the insulationregions, and the semiconductor layer is formed on a top surface andsidewalls of the top portion of the semiconductor region.
 3. The methodof claim 1 further comprising: before the epitaxy, recessing a topportion of the semiconductor region to form a recess, wherein thesemiconductor layer is formed in the recess.
 4. The method of claim 1,wherein the semiconductor layer comprises: a narrow portion; and twowide portions wider than the narrow portion connected to opposite endsof the narrow portion, wherein during the etching the portion of thesemiconductor region, the air gap is formed underlying the narrowportion of the semiconductor layer, and wherein the semiconductor layeris supported by remaining portions of the semiconductor regionunderlying and overlapping the two wide portions of the semiconductorlayer.
 5. The method of claim 1, wherein the forming the semiconductorregion between the two insulation regions comprises: etching a portionof a semiconductor substrate between the two insulation regions to forma recess; and performing the epitaxy to grow the semiconductor region inthe recess.
 6. The method of claim 1, wherein after the forming the gateelectrode, the air gap remains.
 7. The method of claim 1, wherein thegate dielectric and the gate electrode overlap the air gap.
 8. A methodcomprising: performing an epitaxy to grow an epitaxy semiconductor layerover a semiconductor region, wherein the semiconductor region comprisesa middle portion and end portions on opposite sides of the middleportion; etching the semiconductor region to remove the middle portionof the semiconductor region to form a recess, wherein a center part ofeach of the end portions of the semiconductor region remains after theetching; forming a gate dielectric over the epitaxy semiconductor layer;and forming a gate electrode over the gate dielectric.
 9. The method ofclaim 8, wherein during the etching, the middle portion and the endportions of the semiconductor region are etched simultaneously.
 10. Themethod of claim 8, wherein during the etching, the epitaxy semiconductorlayer is exposed to a same etchant as the semiconductor region.
 11. Themethod of claim 8, wherein after the gate dielectric and the gateelectrode are formed, an air gap remains underlying the gate dielectricand the gate electrode.
 12. The method of claim 8, wherein thesemiconductor region is formed in an insulation region, and the methodfurther comprises recessing the insulation region, with a top portion ofthe semiconductor region being over a top surface of the insulationregion.
 13. The method of claim 12, wherein the epitaxy semiconductorlayer is formed on a top surface, and not on sidewalls, of thesemiconductor region.
 14. The method of claim 12, wherein the epitaxysemiconductor layer is formed on a top surface and a sidewall of thesemiconductor region.
 15. A method comprising: performing a first recessto recess Shallow Trench Isolation (STI) regions to have a first topsurface, wherein a top portion of a semiconductor region in the STIregions is over the first top surface; forming an epitaxy semiconductorlayer on a top surface and sidewalls of the top portion of thesemiconductor region; performing a second recess to recess the STIregions to have a second top surface, wherein a bottom end of theepitaxy semiconductor layer is over, and is spaced apart from, thesecond top surface, with an intermediate portion of the semiconductorregion exposed; etching the intermediate portion of the semiconductorregion to form an air gap, wherein the air gap is between the epitaxysemiconductor layer and a remaining bottom portion of the semiconductorregion; forming a gate dielectric over a middle portion of the epitaxysemiconductor layer, with the gate dielectric overlapping the air gap;and forming a gate electrode over the gate dielectric.
 16. The method ofclaim 15, wherein the semiconductor region comprises a middle portionand end portions on opposite sides of the middle portion, with the endportions wider than the middle portion, wherein in the etching, themiddle portion of the semiconductor region over the first top surface ofthe STI regions is fully etched, and the end portions of thesemiconductor region over the first top surface of the STI regions ispartially etched.
 17. The method of claim 15, wherein the epitaxysemiconductor layer and the semiconductor region comprise differentsemiconductor materials, and the etching is performed using an etchantthat does not etch the epitaxy semiconductor layer substantially. 18.The method of claim 15, wherein the epitaxy semiconductor layercomprises silicon, and the semiconductor region comprises germanium,with a germanium percentage of the epitaxy semiconductor layer higherthan a germanium percentage of the semiconductor region, and wherein theetching is performed using an etchant comprising hydrogen chloride(HCl).
 19. The method of claim 15, wherein the epitaxy semiconductorlayer and the semiconductor region comprise different III-V compoundsemiconductors, and wherein the etching is performed using an etchantcomprising H₂O₂ and an acid comprising H₃PO₄ or citric acid.
 20. Themethod of claim 15, wherein the epitaxy semiconductor layer and thesemiconductor region comprise different III-V compound semiconductors,and wherein the etching is performed using an etchant comprising NH₄OH.